RTCAudioSession and RTCAudioSessionConfiguration allow users to handle
audio manually and is used by the AppRTCMobile example.
RTCVideoFrameBuffer exposes a protocol that users can implement to
create their own frame buffer formats, as long as they can be converted
into i420.
RTCVideoCapturer and RTCVideoViewShading are imported by other headers
already included by the umbrella header, so they were always accessible
to users. Added them to the umbrella header to make it explicit.
BUG=webrtc:7351, webrtc:8027
Review-Url: https://codereview.webrtc.org/2994253002
Cr-Commit-Position: refs/heads/master@{#19379}
f9f448b32d